This invention relates to electrochromic devices which can vary the transmission or reflectance of electromagnetic radiation by application of an electrical potential to the electrochromic device.
Certain materials, referred to as electrochromic materials, are known to change their optical properties in response to the application of an electrical potential. This property has been taken advantage of to produce electrochromic devices which can be controlled to transmit optical energy selectively.
A number of factors affect the operation of an electrochromic device. One limitation on how dark an electrochromic device can become is how much charge can be stored in the counter electrode layer. There have been many different approaches for producing a charge storage medium, but most attention has focused on a thin film deposited parallel to the electrochromic material layer, and separated by an ionically conductive layer.
To date, most counter electrode layers have been made using NiO, LiNiO, or doped variants thereof. One advantage of using NiO and LiNiO materials is that under careful preparation conditions, the counter electrode can be made so that it displays anodic electrochromism with good efficiency and a high bleached state transmission. Unfortunately, it has been difficult to intercalate lithium into NiO based materials as a result of the material's compact crystalline structure. As such, higher voltages must be applied to such materials to intercalate lithium, which leads to undesirable side reactions.
Other methods employ proton coloration based mechanisms utilizing counter electrode layers comprised of vanadium oxides and other mixtures containing vanadium. Although it may be relatively easy to manufacture a counter electrode layer capable of coloring anodically in an aqueous medium, it is difficult to produce a complete device capable of long-term stability. It is, therefore, more advantageous to use lithium intercalation based systems.
A typical material used for counter electrode applications with lithium is vanadium oxide, which is a material that forms crystal structures similar to those seen in tungsten oxide systems. The open crystalline lattice of vanadium oxide allows lithium intercalation more readily than in NiO based structures. However, the presence of vanadium ions leads to the generation of a strong yellow color. This yellow color is only slightly modulated by lithium intercalation, and shows a reasonable cathodic electrochromic effect throughout the majority of the visible region, thus limiting the maximum transmission that can be achieved using this material as a counter electrode layer. Attempts to reduce the degree of coloration by doping vandadium oxides with other components result in a reduced electrochromic efficiency by reduction of the charge capacity of the counter electrode layer. Such doping results in a device with a higher bleached state transmission at the cost of a higher colored state transmission.
The problems associated with current counter electrode practice can be summarized with reference to FIG. 1, which provides an illustration of coloration in an electrochromic device having a cathodic counter electrode layer. The overall dynamic range for such a device is given by the net optical density change upon transferring charge from the counter electrode to the electromatic material layer. Such a transfer of charge results in a loss of optical density from the counter electrode and a gain of optical density in the electrochromic material layer. Hence, the net change in optical density is given by the difference in electrochromic efficiency between the electrochromic material layer and the counter electrode. Condition 1 shows a lower overall level of charge and, thus, a lower initial bleached state as compared with Condition 2. On the other hand, Condition 2 shows the situation when the charge capacity is increased, such as by increasing the thickness of the counter electrode layer. As a result, there is an increase in the overall dynamic range along with a concomitant increase in the optical density seen in the bleached state. Therefore, while the total charge of a cathodic counter electrode layer may be increased to enhance the overall dynamic range of the device, the optical density of the counter electrode also increases resulting in a less transparent bleached state.
Devices employing anodic counter electrodes have been briefly discussed in the prior art. Some of these devices employ counter electrodes comprised of nickel oxides doped with tungsten or tantalum. However, the materials comprising such counter electrodes contain the metal oxide in an amorphous phase. As a result, such devices suffer from low coloration efficiencies and low conductivity.
In view of the above problems, there remains a need for improved electrochromic coatings, and in particular electrochromic coatings that comprise solid state, inorganic thin films, and metal oxide thin films. In addition, there remains a need for electrochromic coatings that incorporate complementary anodic and cathodic electrochromic ion insertion layers, whose counter ions are either protons or lithium ions. There also remains a need for improved methods for making complementary electrochromic layers to act as a counter electrode, having improved properties over existing practices. Further, there also remains a need for an electrochromic device with a suitably wide transmission range between fully colored and fully bleached states, with suitably fast coloration and bleaching rates, and with suitable longevity and durability for outdoor architectural applications.